The invention relates to an apparatus for heating a substrate disposed on a substrate holder in a chamber behind a glass plate by the radiation from a lamp disposed outside of the chamber behind the glass plate.
An apparatus of this kind is described for example in U.S. Pat. No. 5,047,611. With it a transparent insulating coating on an integrated circuit is said to be hardened from the inside out because the light first heats the conductor plate and the heat then flows outward from it through the insulating coating. The lamp in this apparatus is disposed at the focal point of a parabolic mirror, so that a parallel beam passes from the lamp through the glass plate to the substrate. In ordinary incandescent lamps the temperature of incandescence of the filament is more than 2000.degree. K. The maximum light emission then has a wavelength of about 1.1 to 1.4 .mu.m. Such short-wavelength light is hardly absorbed by the glass plate, so that the light is able to arrive at the substrate with minor absorption losses.
If transparent substrates instead of opaque substrates are to be warmed in a chamber by externally disposed light sources, it is then necessary to select the wavelength of the radiation so as to obtain maximum absorption of the radiation by the substrate. If, for example, the substrate absorbs at longer wavelengths, the temperature of incandescence of the lamp can be lowered, thereby resulting in a shift of the peak emission toward greater wavelengths. This lowering of the incandescence temperature, however, is bound up with a great decrease of the emission, so that the desired effect of raising the warming rate of the substrate is not achieved. In addition, the shift toward greater wavelengths results in the long-wavelength light also being absorbed to a great extent in the glass plate defining the chamber, so that a greater heat loss occurs at that point. Consequently, in high-vacuum coating apparatus in which transparent substrates are to be coated, the lamps appropriate for this purpose are disposed inside of the vacuum chamber, thereby undesirably increasing the volume of the vacuum chamber.
The problem on which the invention is based is to construct an apparatus of the kind described above such that both opaque and transparent substrates can be warmed therein with the lowest possible energy demand by a lamp disposed externally behind a glass plate.